System for detecting collisions in a shared communications medium

ABSTRACT

A system for detecting collisions in a shared communications medium, such as a TDMA medium, includes a receive path adapted to generate a first intermediate signal, a second intermediate signal, and a data symbol sequence from an input signal. A preamble detection module generates a correlation metric from the first intermediate signal. A power measurement module generates a power indication signal from the second intermediate signal. A noise measurement module generates a noise indication signal from the second intermediate signal and the data symbol sequence. A processing module is adapted to characterize the input signal as a collision for certain values of correlation metric, power indication signal, and noise indication signal.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation of U.S. application Ser. No.09/916,576, filed on Jul. 30, 2001 now U.S. Pat. No. 7,104,534, whichclaims priority to and is entitled to the benefit of Provisional PatentApplication No. 60/296,446 filed Jun. 8, 2001, both of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a communications mediumshared among several users. More particularly, the present inventionrelates to a method and system for detecting collisions in a sharedcommunications medium.

2. Background Art

Communications systems that employ a shared communications medium may,in certain circumstances, permit two or more users to transmitinformation at the same time. For example, many time division multipleaccess (TDMA) communications systems designate one or more “contentiontime slots” within a TDMA frame structure. These contention time slotsare designated for information transmissions from users according tocontention-based communications protocols that allow users to transmit“at will.”

These “at will” transmission protocols can cause multiple transmissionsto overlap in time upon arrival at a designated receiver (for example,at the headend of a Data Over Cable Based Service InterfaceSpecifications (DOCSIS) network). At the receiver, such overlappingtransmissions combine to form a composite signal that is termed acollision. When a collision occurs, the multiple transmissions interferewith each other in a manner that can prevent the reception of a portionor all of the information in these transmissions.

Recovery from a collision requires the retransmission of information byusers. Unfortunately, as collision rates increase, so does theretransmission rate. If the number of retransmissions becomes excessive,latencies associated with the transfer of information increase andcommunications capacity is wasted.

To reduce the amount of wasted capacity, many communications systems areable to adjust their parameters to optimize performance. For example,certain TDMA systems are able to dynamically adjust the number ofallocated contention time slots to keep collision rates, often measuredin collisions per second, within an acceptable range.

Thus, to effectively control collision rates, a communications systemneeds to accurately detect collisions. Conventional collision detectiontechniques do not provide great accuracy. For example, one suchtechnique detects collisions based solely on received signal power.According to this technique, a collision is detected when one or morepower measurements are above a certain level.

This power-based technique disadvantageously assumes that all collidingsignals add in power and, as a consequence, result in a power level thatis greater than a power level associated with a non-colliding signal.Unfortunately, phase and timing offsets between two or more collidingsignals can result in a combined signal that does not have an increasedpower level. In fact, such offsets may also yield combined signalsexhibiting decreased power levels. Furthermore, normal transmit powervariations may corrupt such collision detection processes.

Thus, power-based collision detection approaches fail to detect asubstantial number of collisions. Such failures result in collision rateoverestimation or underestimation, which leads to ineffective contentionslot allocation decisions.

Accordingly, what is needed is a technique to determine with relativecertainty when a collision has occurred in the presence of amplitude,phase, and timing offsets.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to methods and systems for detectingcollisions in a shared communications medium, such as a TDMA medium. Amethod of the present invention includes receiving a signal; calculatinga correlation metric from the signal; measuring a signal noise contentwhen the correlation metric is greater than a first threshold; andclassifying the signal as a collision when the noise content is lessthan a second threshold.

Calculating the correlation metric may include correlating the signalwith a second signal that corresponds to a preamble sequence. Measuringthe signal noise content may include measuring a signal-to-noise ratio(SNR) of a data portion of the signal.

A further method of the present invention includes receiving a signal;generating a correlation metric from the signal, measuring a signalnoise content when the correlation metric is greater than a firstthreshold, and measuring a signal power when the correlation metric isless than or equal to the first threshold. This method classifies thesignal as a collision under certain circumstances. For example, thesignal is classified as a collision when the measured signal power isgreater than a second threshold, and when the measured signal noisecontent is less than a third threshold.

A system of the present invention includes a receive path adapted togenerate a first intermediate signal (e.g., baseband signal), a secondintermediate signal (e.g., a soft decision signal), and a data symbolsequence from an input signal. The system also includes a preambledetection module, a power measurement module, a SNR measurement module,and a processing module.

The preamble detection module is adapted to generate a correlationmetric from the first intermediate signal. The power measurement moduleis adapted to generate a power indication signal from the secondintermediate signal. The SNR measurement module is adapted to generate asignal to noise ratio (SNR) indication signal from the secondintermediate signal and the data symbol sequence.

The processing module is adapted to characterize the input signal. Forexample, the processing module may characterize the input signal as acollision when the correlation metric is greater than a first thresholdand the noise indication signal is less than a second threshold. Inaddition, the processing module may characterize the input signal as acollision when the correlation metric is less than or equal to a firstthreshold and the power indication signal is greater than a secondthreshold.

An advantage of the present invention is the determination of collisionswith greater certainty.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The present invention will be described with reference to theaccompanying drawings. In the drawings, like reference numbers generallyindicate identical, functionally similar, and/or structurally similarelements. The drawing in which an element first appears is indicated bythe leftmost digit(s) in the reference number.

FIG. 1 is a block diagram of an exemplary cable based communicationssystem;

FIG. 2 is a diagram that illustrates an exemplary allocation of timeslots within a TDMA frame;

FIG. 3 is a diagram illustrating an exemplary reservation request;

FIG. 4 is a block diagram of a receiver that detects collisions in ashared communications medium;

FIG. 5 is a diagram of an exemplary signal constellation;

FIG. 6 is a flowchart illustrating a collision detection method;

FIG. 7 is graph that shows preamble gain as a function of both amplitudeoffset and phase offset between two received packets; and

FIGS. 8-10 are graphs showing cases involving the collision of twoexemplary signals.

DETAILED DESCRIPTION OF THE INVENTION I. Operational Environment

Before describing the invention in detail, it is useful to describe anexample environment in which the invention can be used. As the inventionis directed to collision detection in a shared-medium communicationssystem, it is particularly useful in a time division multiple access(TDMA) communications system. For example, the present invention may beimplemented in an upstream communications channel of a cable basedbroadband communications system, such as a DOCSIS network.

FIG. 1 is a block diagram of an exemplary cable based communicationssystem 100 that employs collision detection according to the presentinvention. Communications system 100 includes a master headend 102, hubs104 a-b, nodes 106 a-d, and a plurality of subscribers 108. Subscribers108 exchange bidirectional communications traffic with master headend102 through various optical and electrical media. For instance,communications traffic is passed between master headend 102 and hub(s)104 through optical media, while communications traffic is passedbetween nodes 106 and subscribers 108 through electrical media. Theseoptical and electrical media are described below.

Fiber optic backbone segments 120 a-c provide an interconnection betweenmaster headend 102 and hubs 104. As shown in FIG. 1, backbone segments120 a-c each have exemplary distances of twenty miles or less. However,distances greater than twenty miles are within the scope of the presentinvention.

Nodes 106 each provide an interface between optical communications mediaand electrical communications media. As shown in FIG. 1 fiber opticlines 122 establish connections between hubs 104 and nodes 106. Forexample, fiber optic line 122 d connects hub 104 b and node 106 d. Also,nodes 106 are each coupled to one or more coaxial cables 124. Coaxialcables 124, in conjunction with coaxial cables 126, exchange electricalsignals with subscribers 108. For example, coaxial cable 124 a andcoaxial cable 126 d connects node 106 d with subscribers 108 e and 108f.

Traffic in communications system 100 includes upstream traffic anddownstream traffic. Downstream traffic is received by subscribers 108from system elements, such as master headend 102. In contrast, upstreamtraffic is originated by subscribers 108 and directed to systemelements, such as master headend 102.

For coaxial cables 124, upstream and downstream traffic are eachallocated to a particular frequency band. For example, upstream trafficmay be allocated to a 5-42 MHz frequency band, while downstream trafficmay be allocated to a 54-860 MHz frequency band. One or more frequencychannels exist within these frequency bands that provide for thetransmission of signals. These signals are modulated according to adigital modulation scheme, such as quadrature amplitude modulation (QAM)or quadrature phase shift keying (QPSK).

Multiple subscribers 108 share the electrical and optical communicationsmedia of communications system 100. For instance, in the context ofcoaxial cables 124 and 126, multiple subscribers 108 transmit signalsacross the same frequency channel in the same coaxial cable 124. Toaccommodate such frequency channel sharing, communications system 100employs a multiple access technique, such as TDMA for upstream traffic.

TDMA is a transmission scheme that allows a number of subscribers 108 totransmit information across a single frequency channel withoutinterference. This is enabled by allocating unique time slots to eachsubscriber 108. According to TDMA, subscribers 108 send upstreamtransmissions across a channel during one or more time slots that occurwithin a TDMA frame. Various types of time slots exist. Three examplesare reservation slots, contention slots, and maintenance slots.

Reservation slots are slots assigned to subscribers 108 for thetransmission of application information. Examples of applicationinformation include data traffic and information associated withtelephony applications. Subscribers 108 are assigned reservation slotsin response to upstream reservation requests that they transmit duringcontention slots.

Contention slots are used for the upstream transmission of reservationrequests by subscribers 108. Reservation requests are subscriber 108originated transmissions (also referred to herein as burst transmissionsand packets) that request upstream bandwidth (such as one or morereservation slots) for the transmission of application information. Suchreservation requests are received and processed by one of the systemcomponents, such as a node 106, hub(s) 104, and/or master headend 102.In response, this component includes a controller that transmits adownstream allocation command that indicates allocated upstreambandwidth to the requesting subscriber 108.

During contention slots, any number of subscribers 108 may transmit suchrequests at will. Consequently, multiple requests may interfere or“collide” with each other upon reception at system elements, such asnodes 106. Reservation requests are transmissions having multipleportions. These portions are described below with reference to FIG. 3.

Maintenance slots enable subscribers 108 to perform maintenancefunctions, such as ranging. Ranging is an operation that involvessubscribers 108 each determining the propagation delays incurred insending transmissions across shared media (e.g., coaxial cables 124 and126) to a network element (e.g., nodes 106). This determination allowssubscribers 108 to establish synchronization with the TDMA frame. Inaddition to enabling synchronization, maintenance slots affordsubscribers 108 with the opportunity to perform amplitude and frequencyadjustments.

FIG. 2 is a diagram that illustrates an exemplary allocation of timeslots within a TDMA frame 200. As shown in FIG. 2, a TDMA frame 200 hasa time duration 210 that is divided into a plurality of mini-slots 202.Each mini-slot 202 has a time duration 212 that enables the transmissionof information. TDMA frame 200 may include reservation, contention,and/or ranging time slots that each have durations of one or moremini-slots 202. For instance, the exemplary time slot allocation of FIG.2 shows two allocated reservation time slots 204 a-b, an allocatedcontention time slot 206, and a maintenance time slot 208.

In one embodiment, a reservation request is a packet of symbols, such asQPSK or QAM symbols, having multiple portions. FIG. 3 is a diagramillustrating an exemplary reservation request 300 having three portionstotaling 40 symbols. As shown in FIG. 3, request 300 includes a preamble302, an identification header 304, and a unique data portion 306.

For exemplary reservation request 300, preamble 302 is a predetermined16 symbol sequence. Identification header 304 is an 8 symbol sequencethat contains identification data, such as a medium access control (MAC)header. Unique data portion 306 includes 16 data symbols.

A standard symbol sequence 310 includes preamble 302 and identificationheader 304. Standard symbol sequence 310 is the same for all subscribers108. In contrast to standard sequence 310, unique data portion 306generally differs for each subscriber 108.

During a contention time slot, collisions between two or morereservation requests may occur. These collisions are caused by two ormore reservation requests arriving simultaneously at a designatedreceiver such that portions (or even all) of these transmissions overlapin time. These overlapping portions add and interfere with each other.

The present invention takes advantage of the fact that collidingtransmissions, such as colliding reservation requests 300, have somesymbols in common (e.g., standard symbol sequence 310), and some symbolsnot in common (e.g., unique data portion 306).

II. Exemplary Receiver

FIG. 4 is a block diagram of a receiver 400 that detects collisions in ashared communications medium that employs multiple access techniques,such as TDMA. Thus, receiver 400 may be included in communicationssystem 100 elements, such as nodes 106, hubs 104, and/or master headend102. Receiver 400 includes a demodulation path 460 and a collisiondetection module 470 that is coupled to demodulation path 460.

Demodulation path 460 receives an input signal 420, such as one or moreTDMA burst transmissions, from a shared communications medium (notshown) and extracts an estimated data symbol sequence 432 from it. Inputsignal 420 is an information signal modulated according a modulationscheme, such as QPSK or QAM. In addition, input signal 420 is within acertain bandwidth.

Receive path 460 includes a bandpass filter 402 having a bandwidth thatis greater than or equal to the bandwidth of input signal 420. Bandpassfilter 402 filters input signal 420 into a filtered signal 422 that issent to a low noise amplifier (LNA) 404.

LNA 404 amplifies filtered signal 422 to produce an amplified signal424. Amplified signal 424 is sent to a tuning and sampling module 406,which downconverts amplified signal 424 into a baseband signal. Inaddition, tuning and sampling module 406 samples this baseband signalinto a baseband pulse sequence 426.

A pulse matched filter 408 receives and filters baseband pulse sequence426 to produce baseband signal 428. This filtering operation removesundesired spectral content from sequence 426. Pulse matched filter 408may be a square root raised cosine filter. Baseband signal 428 is sentto a demodulator 410.

Demodulation module 410 demodulates baseband signal 428 according to apredetermined modulation scheme, such as QPSK or QAM. However, othermodulation schemes are within the scope of the present invention. Thisdemodulation process generates a soft decision signal 430.

Slicer 412 maps values of soft decision signal 430 into symbols. Thesesymbols are included in data symbol sequence 432. This mapping functionmay be performed according to adaptive techniques to minimizedemodulation errors.

Collision detection module 470 receives baseband signal 428, softdecision signal 430, and data symbol sequence 432. From these inputs,collision detection module 470 detects the presence of collisions ininput signal 420. As shown in FIG. 4, collision detection module 470includes a preamble detection module 414, a power measurement module416, and a SNR measurement module 418.

Preamble detection module 414 receives baseband signal 428 and processesit to detect the presence of preambles. The presence of a preambleindicates the arrival of a TDMA burst transmission, such as areservation request, as input signal 420. This processing involvescorrelating baseband signal 428 with a baseband preamble signal thatcorresponds to a TDMA preamble sequence.

Through this correlation process, preamble detection module 414generates a correlation metric 434 having a value that indicates thestrength of a preamble sequence. For example, a minimum valuecorrelation metric 434 indicates the absence of a preamble in basebandsignal 428. However, as the value of correlation metric 434 increases,so does the likelihood of a preamble existing in baseband signal 428.

Preamble detection module 414 functionality may be implemented with aprocessor, such as a digital signal processor. Alternatively, thisfunctionality may be provided by matched filters.

By indicating the presence of preambles in baseband signal 428, preambledetection module 414 enables receiver 400 to synchronize itself so thatit may receive and extract information from TDMA burst transmissions. Inaddition, through the generation of correlation metric 434, preambledetection module 414 provides information that enables the accuratedetection of collisions in input signal 420. This utilization ofcorrelation metric 434 is described below in greater detail withreference to FIG. 6.

Power measurement module 416 measures the power of portions of softdecision signal 430. For example, based on TDMA burst synchronizationprovided by preamble detection module 414, power measurement module 416may measure the power in soft decision signal 430 during specificportions of a received burst transmission. For example, with referenceto exemplary reservation request 300, power measurement module 416 maymeasure the power in unique data portion 306. Alternatively, powermeasurement module 416 may measure the power in both identificationheader 304 and unique data portion 306.

Power measurement module 416 may perform such power measurements throughtechniques that involve squaring and summing or averaging values of softdecision signal 430. Such techniques are well known to persons skilledin the relevant arts. Power measurement module 416 generates a powerindication signal 436 that indicates the result of such powermeasurements.

SNR measurement module 418 characterizes the noise content of softdecision signal 430 and generates a SNR indication signal 438 thatincludes this characterization. This noise content may be measuredduring specific portions of a burst. For example, with reference toexemplary reservation request 300, SNR measurement module 418 maymeasure the SNR content in unique data portion 306. Alternatively, SNRmeasurement module 418 may measure the SNR in both identification header304 and unique data portion 306.

SNR measurement module 418 receives soft decision signal 430 andestimated data symbol sequence 432. From these inputs, SNR measurementmodule 418 calculates a signal-to-noise ratio (SNR) of soft decisionsignal 430. SNR measurement module 418 outputs this SNR as SNRindication signal 438.

To help illustrate an SNR calculation technique that SNR measurementmodule 418 may employ, an exemplary signal constellation 500 isillustrated in FIG. 5. Signal constellation 500 corresponds to a 16-QAMmodulation scheme. As shown in FIG. 5, sixteen distinct symbols 502₁-502 ₁₆ are distributed across a signal space defined by an in-phase(i) axis 504 and a quadrature (q) axis 506.

Constellation 500 illustrates a signal 508. Signal 508 is represented bya vector, d (shown as vector 509), which has an in-phase component,d_(i), and a quadrature component d_(q). As described above withreference to FIG. 3, slicer 412 maps values of soft decision signal 430to corresponding symbols within data symbol sequence 432. However, suchmappings reveal noise in soft decision signal 430.

An example of such noise is shown in FIG. 5, where signal 508 isseparated from symbol 502 ₄ by an error vector 510. Error vector 510(also referred to herein as e) has an in-phase component e_(i) and aquadrature component e_(q).

An SNR can be calculated according to Equation (1), below.

$\begin{matrix}{{SNR} = \frac{S}{N}} & (1)\end{matrix}$In Equation (1), S represents the power of signal 508, and N representsthe power of error vector 510. These values are expressed below inEquations (2) and (3).S= |d| ² = d _(i) ² +d _(q) ²   (2)N= |e| ² = e _(i) ² +e _(q) ²   (3)

SNR measurement module 418 performs SNR calculations according toEquations (1) through (3). In performing these calculations, drepresents a symbol from data symbol sequence 432, while e representsthe error vector between this symbol and a corresponding value of softdecision signal 430.

SNR measurement module 418 outputs result(s) of these SNR calculationsas SNR indication signal 438. Alternatively, SNR measurement module 418may output average SNR value(s) as SNR indication signal 438. Thus, SNRmeasurement module 418 may compute the average of a plurality of SNRcalculation results that occur over a specific time window. Suchaveraging computations may employ various weighting schemes.

As shown in FIG. 4, collision detection module 470 includes a collisiondetection module 426 and comparators 450, 452, and 454. Comparators 450,452, and 454 are each coupled to a collision detection logic module 426.In addition, comparator 450 is coupled to preamble detection module 414,comparator 452 is coupled to power measurement module 416, andcomparator 454 is coupled to SNR measurement module 418.

Comparator 450 receives correlation metric 434 and a threshold signal440 having a threshold value T₁. Comparator 450 compares these inputsand generates an output signal 455 that indicates the result of thiscomparison. Output signal 455 may take on distinct values. A first ofthese values indicates that correlation metric 434 is greater than T₁.This first value is referred to herein as “TRUE” since it signifies thedetection of a preamble. A second of these values indicates thatcorrelation metric 434 is less than or equal to T₁. This second value isreferred to herein as “FALSE” since it signifies the absence of apreamble.

Comparator 452 receives power indication signal 436, a threshold signal442 having a threshold value T₃, and a threshold signal 444 having athreshold value T₄. T₄ is greater than T₃. Comparator 452 compares theseinputs and generates an output signal 457 that indicates the result ofthis comparison.

Output signal 457 may take on distinct values. A first of these valuesindicates that power indication signal 436 is greater than T₄. Thisfirst value is referred to herein as “HIGH.” A second of these valuesindicates that power indication signal 436 is less than or equal to T₄and greater than T₃. This second value is referred to herein as “MEDWM.”A third of these values indicates that power indication signal 436 isless than or equal to T₃. This third value is referred to herein as“LOW.”

Comparator 454 receives SNR indication signal 438 and a threshold signal446 having a threshold value T₂. Comparator 454 compares these inputsand generates an output signal 459 that indicates the result of thiscomparison. Like output signals 455 and 457, output signal 459 may alsotake on distinct values. A first of these values indicates that SNRindication signal 438 is greater than T₂. This first value is referredto herein as “HIGH.” A second of these distinct values indicates thatSNR indication signal 438 is less than or equal to T₂. This second valueis referred to herein as “LOW.”

Collision detection logic module 426 receives output signals 455, 457,and 459. Based on the values of these signals, collision detection logicmodule 426 generates a conclusion signal 446. Conclusion signal 446characterizes signal 420. For example, Conclusion signal 446 indicateswhether signal 420 is the result of a collision (i.e., overlapping bursttransmissions), a good packet (i.e., a valid burst transmission), a badpacket (i.e., a corrupted burst transmission), or an empty slot.

Table 1, below, provides an exemplary mapping of output signals 455,457, and 459 to conclusion signal 446. In Table 1, output signal 455indicates whether a preamble was detected in signal 420. Output signal459 indicates the average SNR of the data portion of a bursttransmission, such as unique data portion 306. Output signal 457indicates the power of the data portion of a burst transmission, such asunique data portion 306.

TABLE 1 Output Output Output Conclusion Signal 455 Signal 459 Signal 457Signal 446 True High High Good packet True Low High In-Phase CollisionTrue High Medium Good packet True Low Medium In-Phase Collision TrueHigh Low Good packet True Low Low In-Phase Collision False * HighOut-of-Phase Collision False * Medium Bad packet False * Low Empty slot

As shown in Table 1, conclusion signal 446 can classify input signal 420as either a good packet, an in-phase collision, an out-of-phasecollision, a bad packet, or an empty slot. A good packet is anun-collided packet that is capable of being decoded. An in-phasecollision is a collision between two or more packet conveying modulated(e.g., QPSK or QAM) signals having carrier phases that are substantiallythe same.

In contrast, an out-of-phase collision is a collision between two ormore packet conveying modulated signals having carrier phases that arenot substantially the same. A bad packet is a packet that throughcircumstances, such as attenuation or strong noise conditions, isreceived such that it cannot be decoded. An empty slot denotes theabsence of a packet transmission during a time interval, such as a TDMAslot.

In Table 1, asterisks are placed in the lower three rows of the columncontaining output signal 459 values. These asterisks indicate “don'tcare” conditions. Thus, for the values of output signals 455 and 457that correspond to these rows, output signal 459 generation is notrequired to produce conclusion signal 446.

The elements of receiver 400 may be implemented through varioustechniques, as would be apparent to persons skilled in the relevantarts. For example, these elements may be implemented through electroniccircuitry, and/or digital processing techniques. Such digital processingtechniques may be implemented with a general purpose microprocessor,and/or a digital signal processor (DSP).

III. Operation

FIG. 6 is a flowchart illustrating a collision detection method,according to the present invention. This method enables thecharacterization of signals received across a shared communicationsmedium. This method begins with a step 601. In step 601, a signal isreceived from a shared communications medium, such as a fiber opticbackbone segment 120, a fiber optic line 122, a coaxial cable 124, or awireless or satellite channel. With reference to receiver 400, this stepcomprises receiving input signal 420.

In a step 602, a preamble correlation is generated from the receivedsignal. This step comprises correlating the received signal with apredetermined preamble sequence. This correlation produces a correlationmetric, such as correlation metric 434, which indicates whether thereceived signal includes the predetermined preamble sequence.

When performed by receiver 400, step 602 comprises generating basebandsignal 428 from input signal 420 through the techniques described abovewith reference to FIG. 4. This step further comprises preamble detectionmodule 414 correlating baseband signal 428 with a baseband preamblesignal that corresponds to a preamble sequence in a TDMA burst, therebygenerating correlation metric 434.

In a step 604, it is determined whether the preamble correlation metricis above a first threshold value, such as T₁. If the preamble power isgreater than the first threshold value, then operation proceeds to astep 606. Otherwise, operation proceeds to a step 614. Step 604 may beperformed by comparator 450.

In step 606, signal and noise content is measured. This step maycomprise measuring a data portion SNR. In the context of a TDMA bursttransmission, data portion SNR is an SNR measurement of transmission'sdata portion. For example, with reference to the exemplary reservationrequest 300 of FIG. 3, this data portion may be unique data portion 306and/or identification header 304.

When performed by receiver 400, step 606 comprises generating softdecision signal 430 and data symbol sequence 432 through the techniquesdescribed above with reference to FIG. 4. This step further comprisesSNR measurement module 418 calculating an SNR as described above,thereby generating SNR indication signal 438.

A step 608 follows step 606. In this step, it is determined whether thesignal noise content is above a second threshold value, such as T₂. Ifthe signal noise content is greater than the second threshold value,then a step 610 is performed. Otherwise, a step 612 is performed. Step608 may be performed by comparator 454.

In step 610, the received signal is declared a good packet. Withreference to receiver 400, this step may comprise collision detectionlogic module 426 generating a conclusion signal 446 that characterizesthe received signal as a good packet.

In step 612, the received signal is declared an in-phase collision. Withreference to receiver 400, this step may comprise collision detectionlogic module 426 generating a conclusion signal 446 that characterizesthe received signal as a good packet.

As described above, step 614 follows step 604 if the preamblecorrelation metric generated in step 602 is less than or equal to T₁. Inthis step, a signal power is measured. This step may comprise measuringthe power of a data portion. In the context of a TDMA bursttransmission, data portion power is a power measurement of thetransmission's data portion. With reference to the exemplary reservationrequest 300 of FIG. 3, this data portion may be unique data portion 306and/or identification header 304.

When performed by receiver 400, step 614 comprises generating softdecision signal 430 through the techniques described above withreference to FIG. 4. This step further comprises power measurementmodule 416 generating power indication signal 436.

A step 616 follows step 614. In step 616, it is determined whether thesignal power measured in step 614 is greater than a third thresholdvalue, such as T₃. If so, then a step 620 is performed. Otherwise,operation proceeds to a step 618. Step 616 may be performed bycomparator 452.

In step 618, the received signal is declared an empty slot. Withreference to receiver 400, this step may comprise collision detectionlogic module 426 generating a conclusion signal 446 that characterizesthe received signal as an empty slot.

In step 620, it is determined whether the signal power measured in step614 is greater than a fourth threshold value, such as T₄. If so, a step624 is performed where the received signal is declared an out-of-phasecollision. With reference to receiver 400, this step may comprisecollision detection logic module 426 generating a conclusion signal 446that characterizes the received signal as an out-of-phase collision.Step 620 may be performed by comparator 452.

If it is determined in step 620 that the signal power measured in step614 is less than or equal to the fourth threshold value, then operationproceeds to a step 622. In step 622, the received signal is declared abad packet. Thus, with reference to receiver 400, this step may comprisecollision detection logic module 426 generating a conclusion signal 446that characterizes the received signal as a bad packet.

In embodiments of the present invention, measurements are made in bothsteps 606 and 614 that require the identification of data portions ofTDMA burst transmissions. The data portions of these transmissions areidentified with synchronization information. This synchronizationinformation is based on a timing reference that yields the largestpreamble correlation performed in step 602.

When performed by receiver 400, the operational sequence, describedherein with reference to FIG. 6, provides the mapping of output signals455, 457, and 459 to conclusion signal 446, as shown above in Table 1.This technique enables the detection of collisions with greater accuracythan conventional approaches that are based simply on power.

IV. Threshold Settings

As described above with reference to FIGS. 4 and 6, the presentinvention employs a plurality of thresholds (e.g., thresholds T₁ throughT₄) during the characterization of signals received from a sharedcommunications medium. Exemplary values for thresholds T₁ through T₄ areprovided below. However, these threshold values are intended as examplesand may be adjusted based on operating conditions. Thus, these thresholdvalues may be take on any combination of values.

As described above with reference to FIG. 4, comparator 450 uses T₁ todetect the presence of a preamble in an upstream transmission.Furthermore, T₁ may be used in step 604 of FIG. 6. The power of receivedpackets often varies. In the case of upstream DOCSIS transmissions,variations within a certain tolerance (e.g., ±6 dB) of a nominalpreamble correlation value yield adequate preamble detection.

The nominal preamble correlation value is a preamble correlation metricvalue when a packet is received at an expected power level. Thisexpected power level is referred to herein as nominal power. A receiver,such as receiver 400, may set nominal power to a value within apredetermined power range. For example, in the case of upstream DOCSIStransmissions, nominal power for a 2.56 megasymbols per second (Msps)packet may be in the range of −4 to +26 dBmV.

Once a nominal power value is set, packets may vary within apredetermined tolerance about this value. For example, in a case wherethe nominal power level is set to +10 dBmV, preamble detection using a+/−6 dB tolerance will provide for preamble detection when the preamblecorrelation metric is between +4 and +16 dBmV. Similarly, in a casewhere the nominal power level is changed to +15 dBmV, preamble detectionusing a +/−6 dB tolerance will provide for preamble detection when thepreamble correlation metric is between +9 and +21 dBmV.

Accordingly, T₁ may be set to a value that is substantially equal to thelower end of the preamble detection tolerance range (e.g., 6 dB belowthe nominal power level). Alternatively, T₁ may be set below the lowerend of the preamble detection tolerance range (e.g., 9 dB below thenominal power level).

Comparator 454 uses T₂ to determine whether the data portion of thepacket has adequate SNR. Additionally, T₂ may be used in step 608 ofFIG. 6. T₂ may vary according to the modulation scheme employed forpacket transmission. For QPSK modulation, T₂ may be set to 6 dB, whichcorresponds to the minimum SNR required for QPSK to operate. However,for 16-QAM and higher QAM, T₂ may be set to 9 dB, which corresponds tothe minimum SNR required for QAM to operate.

T₃ is used by comparator 452 to discriminate between the noise floor anda packet. Also, T₃ may be employed in step 620. As described above,packets may have varying received power. In the case of DOCSIS, powerwithin 6 dB of a nominal packet power level may be tolerated. Hence T₃may be set to 6 dB below the nominal packet power level. Alternatively,T₃ may be set to a value beneath the lower end of this tolerance range.For example, in the case of DOCSIS, T₃ may be set to 9 dB below thenominal packet power level.

T₄ is also used by comparator 453 to determine whether two or morepackets collide out of phase, so that their preambles cancel, but resultin their combined unique portions, such as unique data portions 306having high power. Since packets may have varying received power levels,variations (e.g., +/−6 dB in the case of DOCSIS) may be tolerated. Whentwo minimum-power packets collide, the combined data portion power maybe 3 dB below (i.e., −6 dB +3 dB) the nominal packet power level.Accordingly, T₄ may be set substantially equal to 3 dB below the nominalpacket power level. However, T₄, may alternatively be set to a lowervalue, such as 5 dB below the nominal packet power level.

V. Capabilities

The collision detection capabilities of the present invention aredescribed below in the context of various scenarios involving TDMA bursttransmissions (also referred to herein as packets) received from ashared communications medium. In these scenarios, two or more packetsarrive at overlapping times. As described herein, overlappingtransmissions result in a collision.

For these scenarios, the originators of the received packets (e.g.,subscribers 108) have each performed amplitude, time and frequencyadjustment through a ranging process. This ranging process may, forexample, be enabled through one or more TDMA maintenance slots.

Since this ranging process includes amplitude adjustment, the receivedpackets are within a specific amplitude range. For example, the receivedpackets may each have amplitudes that are within 3 dB of each other.However, greater amplitude offsets, such as 6 dB, may occur in certaincases. Furthermore, since the originators of these packets haveperformed timing adjustments, the arrival times of the received packetsare within a certain threshold. For example, the packets may arrivewithin 0.25 symbols of each other.

Unlike amplitude levels and arrival times, the carrier phases of thereceived packets are generally unknown and may be considered to berandom. However, TDMA packets, such as reservation request 300, arerelatively short in duration. Therefore, the received packets may beconsidered to have constant carrier phases over their duration.

Since the arrival times of these received packets are nearly the same,but their phases are random, these packets are referred to herein asbeing “quasi-aligned.” For these quasi-aligned packets, phase andamplitude offsets affect the detectable preamble power upon reception.However, the effect of timing offsets (e.g., within 0.25 symbols) isminor. Therefore, such minor offsets are excluded from this discussion.

In a first set of scenarios, the received packets have identicalamplitudes, but various phases relative to each other. These relativephases are referred to herein as phase offsets. When the phase offset isnear zero, the preambles of the received packets add coherently. Thiscoherent addition results in the detection (e.g., by preamble detectionmodule 414) of a more powerful preamble than if no collision hadoccurred.

For two exemplary received packets having identical amplitudes, theirphase offset affects preamble detection in a manner that is expressedbelow in Equation (1). Equation (1) expresses the gain of the preamblein dB vs phase offset when two equal preambles collide. This gain isanalogous to a simple mathematical model where two vectors of equallength and different phase are summed.gain_dB=20*log 10(abs(2*cos(phase_offset_(—) deg/2*pi/180)))  (1)

Table 2, below, provides preamble power increases for correspondingphase offset values according to Equation (1).

TABLE 2 Phase Offset Between Preamble Power Colliding Packets Increase(phase_offset_deg) (gain_dB)  0 degrees 6.0 dB  30 degrees 5.7 dB  60degrees 4.8 dB  90 degrees 3.0 dB 120 degrees 0 dB 150 degrees −5.7 dB180 degrees −infinity dB

Table 2 shows that, for two colliding packets of equal power, thedetected preamble power is increased two thirds of the time (that is,for phase offset angles from 0 to 120 degrees). In fact, for one-half ofthe time (offset angles 0 to 90 degrees) preamble power is increased by3 dB or more. However, for one-third of the time (offset angles 120 to180 degrees), detected preamble power decreases.

A second set of scenarios involves the received packets having differingamplitudes and differing phases. For this set of scenarios, FIG. 7 isgraph that shows preamble gain as a function of both amplitude offset702 and phase offset 704 between two received packets. Accordingly, FIG.7 includes contours 706 of constant preamble gain.

By scanning across the horizontal axis that corresponds to 0 dBamplitude offset in FIG. 7, the preamble gain values of Table 2 appear.However, as shown in the upper-left corner of FIG. 7, the preamble powerresulting from a collision can be greater than 9 dB when the phaseoffset is small and the amplitude offset is large (e.g., 6 dB andgreater).

In contrast to these preamble detection gains, the middle-right edge ofFIG. 7 shows that the ability to detect a preamble can effectivelydisappear (e.g., preamble attenuation of 20 dB and greater). This trendculminates in complete destructive interference upon the occurrence of a180-degree phase offset and a zero dB amplitude offset.

The techniques of the present invention, as described above withreference to Table 1 and FIG. 6, enable the detection of collisions withhigh likelihood across the entire range of phase and amplitude offsetsshown in FIG. 7. As such, described below are examples involving thecollision of two QPSK signals, denoted signal 1 and signal 2.

Signals 1 and 2 each convey a reservation request 300. However, signals1 and 2 each have different unique data portions 306. QPSK signals aremathematically represented as complex signals having real and imaginaryparts. However, the illustration of the combining of signals 1 and 2provided graphically herein is with reference to only their real parts.Nevertheless, the same principles apply for their imaginary parts.

Signals 1 and 2 each include a plurality of symbols. Before scaling uponreception, the real part of these symbols may be, for example, either +1or −1. However, after scaling (due to amplitude offsets) and phaserotation (due to phase offsets), the real part of signal 1 symbols maybe either a or −a, and the real part of signal 2 symbols may be either+b or −b. Consequently, when combining signals 1 and 2, the resultantsignal will have the quaternary values of a+b, a−b, b−a, and −a−b. Thisresults in the effective randomization of the symbols of signals 1 and2. A comparison of these quaternary symbols to the correct symbol valueswill generally result in a low SNR being measured.

FIGS. 8-10 are graphs showing exemplary cases involving the addition ofsignals 1 and 2. As shown in these graphs, signals 1 and 2, as well astheir combined sums (denoted as “Sum 1+2”), are mapped to portions ofreservation request 300. In these cases, signals 1 and 2 each have anSNR that is on or about 30 dB, which is a high SNR for QPSKtransmission.

The first example, shown in FIGS. 8A-8C, illustrates signals 1 and 2colliding with equal amplitude and phase. In particular, FIG. 8A showssignal 1, FIG. 8B shows signal 2, and FIG. 8C shows the sum of signals 1and 2. FIG. 8C illustrates that preamble 302 and identification header304 add coherently since they convey identical symbols. This coherentaddition enables the detection of strong preamble 302 and identificationheader 304 portions.

In contrast with the ability to detect strong preamble 302 andidentification header 304 portions, FIG. 8C shows that the interval ofthe sum of signals 1 and 2 that correspond to unique data portion 306are effectively randomized. This effective randomization results in alow SNR, which may be measured by SNR measurement module 418. Normalpackets (without collision) generally provide a high SNR when a strongpreamble is detected. However, embodiments of the present inventiondetect a collision when such a low SNR is measured.

The second example, shown in FIGS. 9A-9C, illustrates signals 1 and 2colliding with equal amplitude and opposite phase. In particular, FIG.9A shows signal 1, FIG. 9B shows signal 2, and FIG. 9C shows the sum ofsignals 1 and 2. FIG. 9C illustrates that preamble 302 andidentification header 304 add destructively, virtually canceling tonearly zero. In the context of receiver 400, this will result inpreamble detection module 414 generating a low correlation metric 434value.

In contrast, FIG. 9C further illustrates that unique data portion 306remains strong. Thus, the sum of signals 1 and 2 have a high power levelduring unique data portion 306. In the context of receiver 400, thiswill result in power measurement module 416 generating a powerindication signal 436 that indicates a high power level for unique dataportion 306. Based on the presence of a high power level for unique dataportion 306, embodiments of the present invention conclude that acollision has occurred.

Unlike the present invention, conventional approaches would concludefrom the absence of preamble 302 power in FIG. 9C that no packet hasbeen transmitted. Thus, by failing to detect a collision, suchconventional approaches are unable to register accurate collisionstatistics.

The third example, shown in FIGS. 10A-10C, illustrates signals 1 and 2colliding with a 3 dB amplitude offset and opposite phase. Inparticular, FIG. 10A shows signal 1, FIG. 10B shows signal 2, and FIG.10C shows the sum of signals 1 and 2. FIG. 10C illustrates that preamble302, identification header 304, and unique data portion 306 adddestructively, but do not completely cancel.

As shown in FIG. 10C, unique data portion 306 of summed signals 1 and 2remains strong. However, this portion would exhibit a low SNR because itincludes “randomized” symbols. If the weak preamble (resulting fromdestructive addition of the two transmitted preambles) is detected, thelow SNR in the unique data indicates a collision. If the preamble is soweak as to not be detected, the presence of strong energy in the uniquedata indicates a collision.

VI. Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation.

For example, the present invention is not limited to cable modemsystems. For instance, the present invention may be employed by wirelesscommunications systems, satellite communications systems, and opticalcommunications systems. Furthermore, the present invention may employmodulation techniques other than QPSK and QAM. In addition, the presentinvention is not limited to TDMA. For example, code division multipleaccess (CDMA) systems and orthogonal frequency division multiple access(OFDMA) system may use the present invention.

It will be understood by those skilled in the art that various changesin form and details may be made therein without departing from thespirit and scope of the invention as defined in the appended claims.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A system for detecting collisions in a shared communications medium,comprising: first means for generating a first intermediate signal, asecond intermediate signal, and a data symbol sequence from an inputsignal; second means for generating a correlation metric from the firstintermediate signal and a predetermined preamble sequence; third meansfor generating a power indication signal from the second intermediatesignal; fourth means for generating a SNR indication signal from thesecond intermediate signal and the data symbol sequence; and fifth meansfor characterizing the input signal as a collision or a non-collisionbased on two or more of the correlation metric, the power indicationsignal, and the SNR indication signal.
 2. The system of claim 1, whereinthe fifth means is configured to characterize the input signal as acollision when the correlation metric is greater than a first thresholdand the SNR indication signal is less than a second threshold.
 3. Thesystem of claim 1, wherein the fifth means is configured to characterizethe input signal as a collision when the correlation metric is less thanor equal to a first threshold and the power indication signal is greaterthan a second threshold.
 4. The system of claim 1, wherein the firstintermediate signal is a baseband signal generated by filtering abaseband pulse sequence.
 5. The system of claim 1, wherein the secondintermediate signal is a soft decision signal generated by demodulatinga baseband signal.
 6. The system of claim 1, wherein the input signalcomprises one or more transmissions received from a time divisionmultiple access (TDMA) medium.
 7. A system for detecting collisions in ashared communications medium, comprising: means for correlating areceived signal with a second signal to obtain a correlation metric;means for measuring a signal to noise ratio (SNR) of the received signalwhen the correlation metric is greater than a first threshold; and meansfor classifying the received signal as an in-phase collision when themeasured SNR is less than a second threshold.
 8. The system of claim 7,further comprising means for measuring a power of the received signalwhen the correlation metric is less than or equal to the firstthreshold.
 9. The system of claim 8, further comprising means forclassifying the received signal as an empty slot when the measuredreceived signal power is less than or equal to a third threshold. 10.The system of claim 9, further comprising means for classifying thereceived signal as an out-of-phase collision when the measured receivedsignal power is greater than the third threshold and a fourth threshold.11. The system of claim 10, further comprising means for classifying thereceived signal as a bad packet when the measured received signal poweris greater than the third threshold and less than or equal to the fourththreshold.
 12. The system of claim 11, further comprising means forclassifying the received signal as a good packet when the measured SNRis greater than the second threshold.
 13. The system of claim 7, whereinsecond signal corresponds to a predetermined preamble sequence.
 14. Thesystem of claim 7, wherein said means for measuring SNR measures a SNRof a data portion of the received signal.
 15. The system of claim 7,wherein said means for measuring power measures a power of a dataportion of the received signal.
 16. The system of claim 7, wherein saidreceived signal is from a time division multiple access (TDMA) medium.17. A system for detecting collisions in a shared communications medium,comprising: a receive path configured to generate a first intermediatesignal, a second intermediate signal, and a data symbol sequence from aninput signal; a preamble detection module configured to generate acorrelation metric from the first intermediate signal and apredetermined preamble sequence; a power measurement module configuredto generate a power indication signal from the second intermediatesignal; a signal to noise ratio (SNR) measurement module configured togenerate a SNR indication signal from the second intermediate signal andthe data symbol sequence; and a processing module configured tocharacterize the input signal as a collision or a non-collision based ona logical combination of the correlation metric, the measured signalpower, and the SNR.
 18. The system of claim 17, wherein the processingmodule is configured to classify the received signal as an in-phasecollision when the correlation metric is greater than a first thresholdand the SNR is less than or equal to a second threshold, and themeasured signal power is greater than a third threshold.
 19. The systemof claim 17, wherein the processing module is configured to classify thereceived signal as a collision when the correlation metric is greaterthan a first threshold, the SNR is less than or equal to a secondthreshold, and the measured signal power is greater than a thirdthreshold and less than or equal to a fourth threshold.
 20. The systemof claim 17, wherein the processing module is configured to classify thereceived signal as a collision when the correlation metric is less thanor equal to a first threshold, and the measured signal power is greaterthan a second threshold.